A plastic optical fiber(POF) made of a plastic core and a plastic cladding has been widely used for short distance optical transmission in which the higher optical loss therethrough as compared with a glass optical fiber does not pose a serious problem, a plastic optical fiber having the advantage that it can generally be produced at a lower cost than a glass optical fiber. Depending on the signal detecting type, plastic optical fiber is classified into single mode optical fiber, multi mode step-index(SI) optical fiber and multi mode graded-index(GI) optical fiber.
A graded index plastic optical fiber(GI POF) has a refractive index distribution which decreases gradually from the center of the optical fiber toward the outer periphery thereof. Due to such refractive index distribution, the light transmission speed at the center thereof is low, and high at the outer periphery, but such difference in the light transmission speed is cancelled out by the fact that the path difference at the outer region is longer than at the center. That is, incident signals can be detected simultaneously without delay. Therefore, a GI POF is suitable for high-speed, high-capacity transmission over a wide bandwidth and for short distance optical transmission.
There have been proposed a number of processes for producing a preform for plastic optical fibers, e.g., continuous and batch extrusion processes, but it is difficult to produce plastic optical fibers having a refractive index gradient by continuous extrusion. Therefore, thermal drawing of a preform is generally used for the production of plastic optical fibers having a refractive index gradient.
The most commonly used method for producing a GI plastic optical fiber is interface gel polymerization, wherein a non-reactive refractive index controlling agent is dispersed in a preform in such a manner that the concentration thereof becomes higher as it goes to the center of the preform, to create a refractive index gradient.
The interface gel polymerization is a simple process for producing a GI plastic optical fiber but it is difficult to control the heat of reaction generated by the bulk polymerization, which results in micro pores. This process is, therefore, not suitable for producing a plastic optical fiber whose diameter is 20 mm or bigger.
Japanese Laid-Open Patent publication No. 07-027928 and No. 08-304634 disclose a GI plastic optical fiber comprising many polymer resin layers having different refractive indices, which includes a core having a decreasing refractive index gradient from the center of the core and a clad coated around the core with a lower refractive index than that of the core. However, such a fiber is produced through a complicated production process and has a difficulty in precisely controlling the refractive index distribution. U.S. Pat. No. 5,639,512, No. 5,916,495 and No. 5,614,253 disclose a process of producing a preform of a GI type optical fiber by injecting a polymeric solution into a rotating tube, or by dipping a polymer rod in a polymeric solution containing a refractive index controlling agent to form a polymer coat on the polymer rod, followed by removing the solvent from the coat. But such a process needs additional equipments and process steps for injecting a solution and removing the solvent, although it has the advantage that the refractive index can be easily controlled by adjusting the amount of the refractive index controlling agent added.